Abstract: To improve the heat transfer capability and the crystallization property of the traditional mold flux, CaF₂ was replaced with B₂O₃. Then, the influences of CeO₂ on the heat transfer and the crystallization of the CaF₂-bearing mold flux and the new mold flux with 10wt% B₂O₃ were studied using a slag film heat flux simulator and X-ray diffraction (XRD). The results revealed that the addition of CeO₂ reduced the heat transfer by increasing the solid slag thickness and the crystallization of two mold fluxes. However, CeO₂ had less effect on the B₂O₃-containing mold flux compared with the CaF₂-bearing mold flux. According to the analyses, the CeO₂ contents in the CaF₂-bearing mold flux and the B₂O₃-containing mold flux should not exceed 8wt% and 12wt%, respectively. Therefore, these experimental results are beneficial to improve and develop the mold flux for casting rare earth alloy steels.